Process of cleaning and coating aluminum



Filed April 8, 1954 amm TOE

Inl-Huf :Nm/roza JACK J. BuLLoFF BY 67' ff/L4M' ATTORNEYS United States Patent O PROCESS F CLEANING AND COATING ALUMINUlVI Jack J. Bullo, Dayton, Ohio, assigner to The Common- Wealth Engineering Company of Ohio, Dayton, Ohio, a corporation of Ohio Application April 8, 1954, Serial No. 421,749

4 Claims. (Cl. 117-50) This invention relates to the production of oxide-free aluminum and has -for a primary object the production of aluminum components in such form that advantage may be taken of the chemical activity of the aluminum in such processes as brazing, soldering and welding.

Aluminum from the electromotive point of view should rival calcium in chemical activity, but in reality it is a stable industrial material suitable for structural and domestic purposes and not subject to ready chemical attack.

The inertness of aluminum stems from the adhesion of a protective oxide surface layer of molecular order which renders aluminum bodies passive to chemical reaction and accordingly the material resists brazing, welding and so forth -because the oxide layer remains.

lt is to be noted that stripping of the oxide layer with lye, for example, in the presence of metallic mercury to amalgamate the surface before oxidation thereof sets in, results in the provision of a transient mercury shield which will function in such manner as to permit the aluminum to displace hydrogen from water just as the alkali metals do, but the transient nature of a mercury shield militates against its commercial use.

The present invention contemplates the provision of a metallic shield of a stable nature on oxide-free aluminum bodies; the shield of invention is metallic but may be selected as to its particular nature to adapt the product to predetermined usages.

The invention further contemplates the provision of novel processes for the deposition of metallic coatings on oxide free aluminum, and particularly contemplates the provision of a process which provides over-coats of metal not normally adherent to aluminum. This latter operation is effected by first depositing an under-coat or tieply of an adherent metal on the oxide free aluminum and then adhering the over-coat to the tie-ply.

Another object contemplated *by the invention is the provision of a system for the continuous plating of oxidefree aluminum with a metal or metals, which system includes means for the removal of the oxide of the usual aluminum artifact continuously.

In the inventive process the aluminum is first stripped of any oxide thereon; where the aluminum is in solid form this removal may be effected most readily by passing the aluminum into a bath composed of fused cryolite which acts quickly to remove the usual oxide film and to provide pure aluminum. Where the aluminum is in the molten state and oxide-free it may be maintained in that state by providing suitable equipment to surround it with inert oxygen-free gases prior to and during the operations of coating with a protective metal.

The yfree aluminum preferably under high lvacuum conditions or in an atmosphere of an oxygen free gas which is inert to aluminum is exposed to a salt of a metal which reacts with the aluminum to form a gaseous compound of aluminum and a deposit of the metal of the salt; alternatively a reaction between a vaporized salt of a metal in a gas may be carried out in the presence of the oxide-free aluminum to form a metallic deposit on the aluminum 2,898,230 Patented Aug. 4, 1959 ICC and a volatilized gaseous compound which is continuously removed `from the area of the aluminum as the plating action progresses.

The metals deposited as described above if adhered to aluminum and inert to the atmosphere may provide the only protective medium, or if the over-coat metal is itself of such a nature as to be non-adhering to aluminum a tie-ply may first be deposited in accordance with the above indicated reactions.

Metals which are suitable as either an under-coat or an over-coat for the oxide-free aluminum include osmium, molybdenum, gallium, beryllium, iron, rhenium, tellurium, antimony, bismuth and gold, for example, all of which are derivable from their chlorides.

Metals which are particularly suitable as over-coats and for adherence to the aforementioned are titanium, rhenium, tantalum, iridium, tungsten, boron, silicon, all of which are derivable from their fluorides or chlorides and the metal of which is preferably deposited by reaction of the salt with hydrogen in the presence of the oxidefree aluminum. Zirconium and hafnium are also derivable from their chlorides with hydrogen in the same manner and are similarly effective.

The compounds from which the metals are derived in the practice of the invention are volatilizable materials and reaction, for example, between the compound and the aluminum gives rise to volatile compounds of aluminum which are withdrawn from the plating operation as they are formed; similarly volatile compounds are formed in the practice of the invention when vapors o1' gaseous constituents react to deposit on the aluminum a metal, and these likewise are removed from the plating area as they form; thus the plate is not contaminated.

It is to be noted that gaseous compounds containing oxygen and frequently useful in the deposit of metals such as the carbonyls and the nitrosyls are not useful in connection with the practice of this invention as they lead to the presence of materials containing carbon and oxygen and the tendency of the oxide-free aluminum surface is to react with either carbon or oxygen to form carbides or oxides.

The invention will be more fully understood by reference to the following detailed description and accompanying drawings wherein:

Figure 1 is a schematic View of apparatus useful in the practice of the invention; l

Figure 2 is a -sectional view illustrating the product of invention;

Figure 3 is a sectional view illustrating another product of the invention; and

Figure 4 is a sectional view illustrating apparatus which is useful in the practice of a further embodiment of the process of invention.

Referring to the drawings there is shown at 1 in Figure l a roll of aluminum sheet 2 rotatably supported on a stand 3 and adapted to be drawn by rollers 5, 7 one of which is preferably driven towards the right in Figure l. Shown at fthe far right in the figure is a reel 9 supported at 11 and upon which the aluminum sheet is wound after treatment in accordance with the inventive process. The reel is for this purpose driven through a gear box 13 and a motor 15 and the speed of the roller 5, for example, may be synchronized with that of the motor in any convenient manner as by use of variable speed belting.

Shown at 17 is a tank containing molten cryolite i9 which is heated inductively by means of coil 21, or if desired the cryolite may simply be heated by a gas flame. -The aluminum sheet 2 passes over pulley 23 and is directed downwardly around pulleys 25, 27 through the cryolite bath. This cryolite quickly dissolves aluminum oxide which is usually present on exposed surfaces of aluminum and most suitably the bath contains for this 3 purpose non-oxygenic salt such as luorspar (Ca`F2) which assists in rendering the temperature of the molten cryolite relatively low and prevents any tendency of the aluminum to become surface hardened in the course of its passage through the bath. Accordingly an oxide-free aluminum strip is passed outwardly out of the cryolite over pulley 29.

The bath functions as a seal and is covered by a suitable housing 31 having a depending portion 33 which extends into the bath 19 to provide a seal for the interior of the plating apparatus indicated at 35 from the atmosphere.

The construction of the plating compartments and the electrical connections of the apparatus are shown specically in Patent No. 2,580,976, issued January l, 1952, to H. A. Toulmin, Jr. The apparatus includes supporting rollers 37 for the aluminum strip; also upper rollers 39 are supported from plates 40 which divide the apparatus into insulated compartments, and these upper rollers are each provided with brushes 41 of any suitable type which bear against the rollers and pass electric current thereto from a source (not shown) if additional heat is necessary.

The rollers contact the aluminum strip and current flows through `the aluminum section of each compartment to heat the aluminum strip portion therein; in the present case the temperature of the aluminum is preferably somewhat less than 500 F., for example 450 F., and employment of the electrical heating is not necessary, the residual heat from the cryolite being sufficient.

The compartments formed in the apparatus are indicated at 45, 47, 49, 51, 53 and 5S. The aluminum first passes into compartment 45 which has prior thereto been completely exhausted of air and which is connected to a vacuum pump (not shown) to provide for continuous exhaustion of the chamber and for the removal of any gases which may come from the cryolite bath itself.

Further, there is provided in the upper end of this chamber a port S8 through which deoxygenated argon may be passed from the port 57 under the influence of the vacuum pump. Accordingly the atmosphere within the chamber 45 will be completely static with respect to the aluminum passing therethrough.

Chamber 47 may also be maintained under high vacuum conditions, gases being withdrawn therefrom through valve 59 as indicated. If desired this chamber may also have oxygen-free argon fed to it through inlet 60 and thus it is assured that no oxygen Iwill contact the aluminum in its passage from the plating compartment 49,

Plating compartment 49 is provided with an inlet 6i and an outlet 62 and in the present case vapors of antimony chloride (SbCl3) which has a boiling point of about 219 C. are passed therethrough. The antimony derivable frorn this compound may itself form the over-coat, but in the present instance, for the purposes of clearly' explaining the invention, it will be assumed that the antimony is being deposited from the vapor state as an under-coat or tie-ply.

The vapors of the antimony chloride are fed to the chamber 49 through the inlet 61 and when they contact the heated aluminum they will react therewith to produce vapors of aluminum chloride and a deposit of antimoney on the metal. The vapors of aluminum chloride are withdrawn together with any undecomposed vapors of antimony chloride under vacuum pressure through conduit 62. These vapors may be condensed and the constituents thereof recovered if desired.

The antimony coated aluminum is then passed successively into chambers 51 and 53 each of which is provided with an atmosphere comprised of molybdenum, chloride and hydrogen, the molybdenum entering through ports indicated at 63, 64 and the hydrogen entering at 65, 66. In each of these chambers the molybdenum chloride, which volatilizes at about 268 C. reacts with the hydrogen gas in the vapor state to produce hydrogen chloride vapors and metallic molybdenum, which latter deposits on the slowly moving strip of antimony coated aluminum.

The vapors of hydrogen chloride are withdrawn through conduits 67, 68 under vacuum conditions of the reaction area and the strip itself passes on to chamber 55 and therethrough for winding at reel 9.

The end chambers of the apparatus 45, 55 are provided with seals 70, 72, more fully described in the patent referred to hereinbefore, to prevent the ingress of any oxygen to the surface being plated. The central compartments, that is 49, 51, 53, are preferably maintained at very low vacuum conditions, the preferred pressure for the molybdenum reaction being about one millimieter of mercury. If desired, however, inert de-oxygenic gas may be fed into these chambers to create a higher pressure and to assist in maintaining the internal atmosphere free of oxygen.

As further examples of the treatment of invention the chamber 49 may be supplied, for example, with bismuth chloride and in this case the atmosphere internally should be maintained at as low a pressure as is obtainable, preferably only a few microns of mercury. Chambers 51, 53 may `be then supplied with titanium tetra-chloride which volatilizes at about 136 C. and which deposits on reaction with the hydrogen, supplied through conduits 65, 66, titanium metal on the bismuth.

It is to be noted with respect to molybdenum deposits on aluminum that the same may be attained directly by feeding molybdenum chloride only into the apparatus to react the same with the oxide-free aluminum and produce aluminum chloride and a molybdenum deposit on the solid aluminum body remaining.

In the practice of invention it is only necessary that the aluminum is fed through the apparatus described or other suitable apparatus, but kept at a. temperature consistent with the results desired. For example, in the case of a strip which is to retain its strip form the temperature must not reach the softening point of aluminum. However, molten aluminum or aluminum which is in a somewhat plastic condition may be plated in accordance with the precepts of the invention by utilizing apparatus such as that illustrated in Figure 4 and more particularly shown in Patent No. 2,657,457, issued November 3, 1953, to H. A. Toulmin, Ir.

Thus as shown in Figure 4 a hot body of aluminum 7S may be passed through a plating unit indicated generally at 77 which comprises an inner plating chamber 78, an outer annular space 79 through which inert gas continually passes, and an intermediate annular space 80 which is generally under a higher pressure than that in either the inner chamber or the outer annular space. ln this instance the vapors carrying the gas to be plated enter the apparatus through conduit 81 and the gases to be withdrawn pass out under vacuum conditions through conduit 82.

The apparatus effectively prevents leakage and the aluminum body 75, as indicated in the patent mentioned, maybe a body which has thus been formed from molten aluminum and is accordingly oxygen-free and is protected in its passage to the plating chamber by surrounding the body with, for example, a hydrogen atmosphere.

The products of the inventive concept are indicated in 4section in Figures 2 and 3 wherein the oxygen-free aluminum body is indicated at 83 and 83a, respectively. In Figure 2 the over-coat 84 may be any of the metals which is adhered to aluminum, which is, as indicated hereinbefcre, for example, molybdenum.

In Figure 3 the aluminum body 83a may be provided with an under-coat 85 with antimony as already described and the overcoat may be any suitable metal such as molybdenum 87.

Metals described as over-coats and under-coats may be of such a nature as to be readily Welded, brazed or soldered themselves or the inventive body may of course be used for other purposes wherein available oxygenfree aluminum is desired. In any event the under-coat gil Wi.

and over-coat will be selected with a view to the ultimate purpose for which the product is designed.

Other components, combination of components, ternperatures and pressures may be utilized in the practice of the invention and the illustrative examples set forth hereinbefore are not to be considered limitative of the inventive concept.

It is to be noted with respect to Figure l mode of operation that cryolite is higher melting than aluminum. The aluminum sheet or Wire or other body which passed into the melt has to pass -at a rate sufficiently high to prevent melting of aluminum metal, but suiciently slowly to remove the oxide effectively. This speed varies 'with conditions and particularly the thickness of the aluminum cross-section, the thicker sections conducting heat away from the portion in the bath more readily. The addition of fluorspar, common salt, or aluminum chloride may be employed to lower the bath temperature if necessary to about 850 C. In the event that operating conditions result in a frozen salt coat on the aluminum, provision of squeeze rollers or knife-edge guides at 29 (Figure 1) will strip oi the brittle non-adherent coating easily. Any tendency of the coating to adhere indicates that the pass rate is too high, and strip movement may be slowed accordingly to prevent such adherence.

It will be understood that this invention is susceptible to modiiication in order to adopt it to different usages and conditions and accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claims.

I claim:

1. The process of providing a metal coated, oxidefree aluminum body which comprises subjecting a body of aluminum having an oxide coating thereon to fused cryolite to remove the said oxide coating and to leave a body of substantially pure aluminum, contacting said oxide-free aluminum body with vapors of an oxygenfree metal bearing compound while providing the aluminum at a temperature to react with the metal bearing compound to produce a volatile aluminum compound and a metal deposit of the metal of the compound on the aluminum body, removing the volatile aluminum compound as it is formed to provide an aluminum body having an adherent coating of metal thereon which is resistant to oxidation and which inhibits oxidation of the aluminum body.

2. The process of providing a metal coated, oxide-free aluminum body which comprises `subjecting a body of aluminum having an oxide coating thereon to a bath of fused cryolite to remove the said oxide coating without melting olf the aluminum body and to leave a body of substantially pure aluminum, contacting said oxidefree aluminum body with vapors of an oxygen-free metal bearing compound While providing the aluminum body at a temperature to react with the metal bearing compound and by a displacement reaction to produce a volatile aluminum compound and a metal deposit of the metal of the compound on the aluminum body, removing the volatile aluminum compound as it is formed to provide an aluminum body having `an adherent coating of metal thereon which is resistant to oxidation and which inhibits oxidation of the aluminum body.

3. The process of providing a metal coated, oxidefree aluminum `body which comprises subjecting aluminum in the form of an elongated body having an oxide coating thereon to a bath of fused cryolite to remove the said oxide coating without melting of the aluminum body and to leave a body of substantially pure aluminum, contacting said oxide-free aluminum body with vapors of an oxygen-free volatile chloride of a metal by passing the body through the vapors while providing the aluminum body at a temperature to react with the chloride of the metal to produce a volatile aluminum chloride and a metal deposit of the chloride on the aluminum body, removing the volatile aluminum chloride as it is `formed to provide an aluminum body having an adherent coating of metal thereon which is resistant to oxidation and which inhibits oxidation of the aluminum body.

4. The process of providing a metal coated, oxidefree aluminum body which comprises subjecting a body of aluminum in the form of an elongated sheet having an oxide coating thereon to a bath of fused cryolite to remove the said oxide coating without melting the aluminum body and to leave a body of substantially pure aluminum, contacting said oxide-'free aluminum body with vapors of -antimony chloride by passing the sheet through the vapors while providing the aluminum body at a temperature to react with the antimony chloride to produce volatile aluminum chloride and a deposit of antimony on the aluminum body, removing the volatile aluminum compound as it is formed to provide an aluminum tbody having an adherent coating of antimony thereon which is resistant to oxidation and which inhibits oxidation of the aluminum body, passing the sheet through a plating atmosphere of molybdenum chloride and hydrogen to produce a deposit of molybdenum on the antimony and hydrogen chloride vapors, withdrawing the hydrogen chloride vapors as they are formed, and withdrawing the coated body of aluminum from the plating atmosphere.

References Cited in the file of this patent UNITED STATES PATENTS 

1. THE PROCESS OF PROVIDING A METAL COATED, OXIDEFREE ALUMINUM BODY WHICH COMPRISES SUBJECTING A BODY OF ALUMINUM HAVING AN OXIDE COATING THEREON TO FUSED CRYOLITE TO REMOVE THE SAID OXIDE COATING AND TO LEAVE A BODY OF SUBSTANTIALLY PURE ALUMINUM, CONTACTING SAID OXIDE-FREE ALUMINUM BODY WITH VAPORS OF AN OXYGENFREE METAL BEARING COMPOUND WHILE PROVIDING THE ALUMINUM AT A TEMPERATURE TO REACT WITH THE METAL BEARING COMPOUND TO PRODUCE A VOLATILE ALUMINUM COMPOUND AND A METAL DEPOSIT OF THE METAL OF THE COMPOUND ON THE ALUMINUM BODY, REMOVING THE VOLATILE ALUMINUM COMPOUND AS IT IS FORMED TO PROVIDE AN ALUMINUM BODY HAVING AN ADHERENT COATING OF METAL THEREON WHICH OF RESISTANT TO OXIDATION AND WHICH INHIBITS OXIDATION OF THE ALUMINUM BODY. 